The problem we set out to solve
Capital Gains Tax in the UK has changed more in the last few years than in the decade before it. The annual exempt amount has been cut repeatedly, the rates on residential property differ from the rates on everything else, and reporting deadlines for property are far shorter than most people expect.
The practical consequence is that a lot of the CGT calculators you find online quietly return the wrong answer, because they were built against an older allowance and never revisited. GainCalc was built because we kept finding that out the hard way.
What the calculator does
It estimates the Capital Gains Tax due on a disposal: shares, property, or other chargeable assets. You enter the acquisition cost, the disposal proceeds, allowable costs, and your income for the year. It applies the annual exempt amount, works out how much of the gain falls in the basic rate band and how much above it, and applies the correct rate for the asset type.
It deals with the detail that trips people up: that your other income determines which CGT rate applies, that residential property is taxed at a higher rate than shares, and that allowable costs include acquisition and disposal expenses that many people forget to deduct.
Rates, allowances and how we keep them current
Rates and the annual exempt amount come from HMRC published figures for the current tax year. We review them at each Budget and again ahead of April. Where a rate has changed part way through a tax year, we state which figures the calculator is using rather than leaving you to guess.
Who publishes this site
GainCalc is built and maintained by CJ Software Ltd, a UK software company that develops a small family of free, single-purpose calculators. We are a software business, not a firm of accountants, and we say so plainly because it matters when you decide how much weight to give a number on a screen.
The site is funded by advertising. Advertisers have no influence over the calculations, the guidance, or what we choose to write about.
Limitations worth knowing
The calculator handles a straightforward disposal well. It does not model share matching rules, the thirty day bed and breakfasting rule, Business Asset Disposal Relief, Private Residence Relief apportionment, losses carried forward from earlier years, trusts, or non-UK residence. Any of those can change the answer substantially.
Treat the output as a sense check on what you might owe, not as a filing figure. For an actual return, use a qualified tax adviser or HMRC guidance directly. If you spot an error in our figures, please tell us.